1. Field of the Invention
The present invention relates to a driving circuit for a camera, and more particularly to a driving circuit for the so-called electrically operated camera using drive motors for winding and rewinding a film, charging the shutter and so on.
2. Description of the Related Art
Such a driving circuit for an electrically operated camera generally comprises a central processing unit (CPU) for sequence control and drivers for driving motors. The driver is generally composed of bridge circuits comprising bipolar transistors. In this case a predriver needs to be connected between the CPU and the driver for the following reasons. A battery is commonly used for the power source of the camera and thus there is a need for a CPU of a low current dissipation type. In general, therefore, a CMOS type of CPU is used. With the CMOS type of CPU, its design rule has become increasingly small. Hence it is small in output driving capability, low in output withstand voltage and small in current supply capability at its I/O ports, thereby failing to directly drive the bipolar transistors of the bridge circuits.
A prior art of such a camera driving circuit is shown in FIG. 1. CPU 1 is used for controlling a sequence of overall operations. First and second motors (driving sources of a camera) 2 and 3 are controlled by bridge circuits 4 and 5, respectively. The bridge circuits are each composed of bipolar transistors as shown. Between CPU 1 and bridge circuits 4 and 5 is connected a predriver circuit 6 which is composed of discrete components such as bipolar transistors, resistors, etc. If the predriver is formed of discrete components as described above, when the number of the motors to be controlled increases, the component count will increase accordingly. This will increase the space occupied by a circuit board, the number of steps for assembly and manufacturing cost.
Another prior art is shown in FIG. 2 according to which the component count does not increase even if the motors increase in number. This prior art is described in U.S. Pat. No. 4,772,908. CPU 1 and bridge circuits 4, 5 are the same as those in FIG. 1. Pre-driver 7 is composed of a monolithic IC in place of discrete components. The monolithic IC is of a bipolar type and incorporates a decode circuit for decoding digital signals from CPU 1 and a driver circuit for driving bridge circuits 4 and 5. With the monolithic IC pre-driver incorporating the decode circuit and the driver circuit, the component count does not increase even if the motors to be controlled increases in number. However, a problem with the prior art is that the number of signal lines connected between CPU 1 and pre-driver 7 increases.